1. Field of the Invention
The present invention relates to a device having an element in which a light-emitting material is interposed between electrodes (hereinafter, such a device is referred to as the light-emitting device and such an element is referred to as the light-emitting element). In particular, the present invention relates to a device including on one and the same insulating surface, a pixel section and a driver circuit for transmitting a signal to the pixel section. In addition, the present invention can be used for a device having an element in which liquid crystal is interposed between electrodes (hereinafter, such a device is referred to as the liquid crystal display device and such an element is referred to as the liquid crystal element). It should be noted that in the present specification, the light-emitting device and the liquid crystal display device are collectively referred to as the display device.
Light-emitting materials that can be used in the present invention include all of light-emitting materials that emit light (phosphorescent light and/or fluorescent light) via singlet excitation or triplet excitation, or both of these excitations.
2. Description of the Related Art
Recently, developments for a light-emitting device including a light-emitting element which utilizes a light-emitting material capable of providing EL (Electro Luminescence) has been progressed (hereinafter, such a light-emitting device is simply referred to as the light-emitting device; such a light-emitting element is referred to as the EL element; and such a light-emitting material is referred to as the EL material). The light-emitting device has a structure having an EL element in which a thin film made of the EL material is interposed between an anode and a cathode.
Although in the developments for the light-emitting devices the passive-matrix type devices have been mainly focused, it has been considered that there will exist disadvantages with the passive-matrix type light-emitting devices in that a sufficient reliability (a long lifetime of the EL element) cannot be ensured with a higher precision pixel section which requires the luminance of the EL element to be increased. From the above circumstances, the active-matrix type light-emitting devices are recently drawing much attention for the purpose of realizing a higher precision display. The active-matrix type light-emitting device is characterized in that an active element is provided within each pixel so that the EL element is allowed to emit light in accordance with an input signal. As the active element, a TFT (Thin Film Transistor) is commonly employed.
Reference is now made to FIG. 4, which illustrates a pixel structure of the active-matrix type light-emitting device. In FIG. 4, reference numeral 401 denotes a source wiring, 402 denotes a gate wiring, 403 denotes a TFT functioning as a switching element (hereinafter referred to as the switching TFT), and 404 denotes a capacitor electrically connected to a drain of the switching TFT 403.
The drain of the switching TFT 403 is also electrically connected to a gate electrode of a current-controlling TFT 405. A source of the current-controlling TFT 405 is electrically connected to a current supply line 406, while a drain thereof is electrically connected to an EL element 407. In other word, the current-controlling TFT 405 can function as an element for controlling current flowing through the EL element 407.
The luminance of the EL element can be controlled by thus providing the two TFTs having different functions, respectively, in each of the pixels. As a result, a light-emitting period can substantially correspond to one-frame period, and an image can be displayed while suppressing the luminance even with a higher precision pixel section. Furthermore, advantages of the active-matrix type device include the capability of forming, as a driver circuit for transmitting a signal to the pixel section, a shift register or a sampling circuit with TFTs on the same substrate. This enables fabrication of a very compact light-emitting device.
However, it is difficult to ensure a sufficient production yield of the active-matrix type light-emitting device, as compared to the passive-matrix type device that has a simpler structure, since a plurality of TFTs have to be formed on the same substrate in the active-matrix type device. Particularly in the case where the driver circuit is to be provided on the same substrate, a line defect may arise in which one line of the pixels does not operate because of a defect of operation. In addition, since fabrication steps for the TFTs are relatively complicated, there is the higher possibility of increasing, a fabrication cost of the active-matrix type device, as compared to that of the passive-matrix type device. In such a case, a disadvantage of increasing a price of an electrical apparatus employing the active-matrix type light-emitting device in its display section may arise.
Thus, the present invention is intended to reduce a fabrication cost of the active-matrix type display device so as to provide an inexpensive display device. In addition, the present invention is also intended to provide an inexpensive electrical apparatus that employs in its display section, the display device in accordance with the present invention.